NMR chemical shift spectroscopy has been in use for a relatively long time. For example, in 1950 E. L. Hahn published an article in the Physical Review, volume 80, pp 580 which disclosed a sequence to obtain stimulated echo signals (STE) for use in spectroscopic experiments. In 1973 P. C. Lauterbur in an article published in Nature (London) 242, 89/90 disclosed the use of field gradients for determining the source location of free induction decay (FID) signals obtained in NMR experiments.
It has long been known that when atomic nuclei that have net magnetic moments are placed in a strong static magnetic field, the nuclei ("spins") precess about the axis of the field at the Larmor frequency given by the equation: EQU f=.gamma.Bo/2.pi.
in which:
.gamma. is a gyromagnetic ratio, constant for each NMR isotope which exhibits a net magnetic moment; PA1 Bo is the strength of the magnetic field; and PA1 .pi. is the well known constant 3.1416+. PA1 .DELTA.f is an offset frequency (added to the Larmor frequency); and PA1 .DELTA.F is the bandwidth of the RF pulse.
The angular velocity of the precesion is primarily dependent on the strength of the magnetic field Bo and increases with increasing field strength. The chemical shifts occuring where the Larmor NMR frequency of the nuclei of the same element differ because of different magnetic environments produced by differences in their chemical environment. For example, electrons partially shield the nuclei from the external magnetic field and thereby affect the resonant frequency. The degree of shielding caused by the electrons depends on the chemical environment of the nucleus. Thus, the chemical shift spectrum of a given molecule is unique and can be used for identification purposes. Since the resonant frequency and hence the absolute chemical shift depend on the applied field, the chemical shift spectrum is expressed in fractional parts per million (ppm) of the NMR frequency relative to an arbitary reference compound.
For example, the range of chemical shifts is about 10 parts per million for protons, 30 parts per million for phosphorus-31, and 200 parts per million for carbon-13. In order to discern small chemical shifts the homogeneity of the field Bo must exceed differences in chemical shifts of the peaks in the spectrum and typically must be better than one part in one million.
In conventional NMR spectroscopy, chemical shifted signals are absorbed from the whole of the sample placed in the region to which the NMR coil is sensistive. This is satifactory for studying the chemical structure of homogeneous samples in vitro; however, to enable discrimination of normal and abnormal conditions in medical diagnostic applications in vivo it is necessary to spatially discriminate signal components in the sample. Thus, in diagnostic applications of spectroscopy it is not satisfactory when signals are observed from the whole of the NMR sample. For example where the sample is a part of a human body, the sample is notoriously non-homogeneous. Accordingly, it is necessary to obtain signals from small volumes of interest from within the human body in vivo in the large static magnetic field.
Localization of the volume of interest is critically important for such NMR medical diagnostic studies in vivo. Selection of a cubic volume can be achieved by application of RF pulse sequences comprising three consecutive tailored RF pulses, each in the presence of a different one of the three orthogonal gradients. The use of such pulses sequences such as 90 degrees, 180 degrees and 180 degrees has been reported by R. E. Gordon and R. J. Ordidge, in a report entitled "Volume Selection for High Resolution NMR Studes" in the Proceedings of the SMRM Third Annual Meeting, 1984 at pp 272 et seq. A pulse sequence using a composite pulse such as selective 45 degrees, non-selective 90 degrees and selective 45 degrees with the composite pulse applied three times has been reported in an article by W. P. Aue, S. Muller et al in the Journal of Magnetic Resonance, vol 56 pp 350 et seq. "A Selective Volume Method for Performing Localized NMR Spectroscopy", is the subject of the U.S. Pat. No. 4,480,228 which were issued on Oct. 30, 1984.
The 90-180-180 prior art pulse sequence procedure for spatially localizing the spectroscopic signals received yields signals that are strongly dependent on the T2 relaxation times of the spins that provide the signals. This dependance on the T2 relaxation times makes it difficult to detect signals with short T2 relaxation times, for example, the signals of phosphorus nuclei of ATP, in vivo.
Another problem with the prior art pulses sequence methods for spatially localizing the spectroscopic signal is that the RF power used tends, to heat the tissue of the subject. It is therefore incumbent on the designers of such methods to minimize the RF power deposition.
Yet another problem caused by the employment of 180 degree RF pulses is the poor definition of the selected volume.
The ability to obtain stimulated echoes as previously noted has been known to those skilled in the art for a long time. It is also known that among the benefits obtained by using stimulated echoes in NMR imaging for example, is that no 180 degree pulses are needed. Therefore, when acquiring data using stimulated echoes, the applied power is considerably reduced as compared to the spin echo data acquisition sequences.
In an article entitled "Stimulated Echo Imaging" by J. Frahm, et al which appeared in the Journal of Magnetic Resonance, Vol. 64, pp 81-93, (1985) is was noted that stimulated echo imaging has reduced dependence on T2 relaxation time and that RF power is reduced. However, until now nobody has applied stimulated echo pulse sequences for acquiring spectroscopic data of spatially localized volumes.
Accordingly solutions to the problems of obtaining spatially localized spectroscopic data are still being sought. The desired method should have reduced dependance on the T2 relaxation time in order to detect signals from nuclei with short T2 relaxation times, in vivo. Also, spectroscopic data acquisition methods are being continuously sought wherein lower RF power can be used for acquiring the data. In addition the desired method should provide more precise definition of the volume and eliminate unwanted signals from regions outside of the region of interest.